how to avoid redownloads or skip downloads if file already exists

How can this be done . I have downloaded some files in the folder and download was inturpted ans download was incomplete. i do not want to redownload those multi gigabytes file.I just want to skip files which are already downloaded and download which are not .

I have list of clear text urls which I used to download files . I wish to remove the links of already downloaded files o that I can skip redownloads.